Discharge lasers, especially excimer lasers, are being increasingly used in the photolithographic manufacture of semiconductor devices. As projection lens technology has advanced, the requirements on the control of the laser light output have increased. For example, variations in the output power, central wavelength and wavelength spectrum of a laser source must be minimized to ensure consistent process performance. To achieve this control, the output wavelength and/or spectral bandwidth of the laser must be well characterized, and remain stable and precisely controlled over a wide range of operating conditions.
A typical measurement technique directs a portion of the laser beam through an etalon to create an optical fringe pattern on the surface of a sensor. The spacing of the fringe pattern can be related to wavelength and the thickness of each fringe can be related to spectral bandwidth. Both fringe spacing and thickness can be detected with a sensor such as a multi-element photodiode array (PDA), and the resulting measurements can be input to a wavelength/bandwidth feedback control system.
In more detail, an etalon, e.g. a Fabry Perot interferometer having a plano configuration, is an optical device consisting of two flat surfaces, held parallel to high precision, typically to within a small fraction of the laser wavelength. These surfaces may be, for example, opposed flat sides of a transparent optical element, in which case the optic is referred to as a “solid etalon”. In another type of construction, the etalon could be formed by the adjacent surfaces of two transparent optical elements, separated by a spacer or spacers with parallel faces. This assembly is often referred to as an “air-spaced etalon”, although the gap between the two elements could be filled with any transparent medium.
Special coatings can be applied to the surfaces of the etalon to enhance their reflectivity at a particular wavelength, or range of wavelengths. This creates an optical cavity, in which constructive and destructive interference of light passing through the cavity can occur. The nature of this interference will depend, among other factors, upon the wavelength, spectrum, and direction of the light, the flatness, parallelism and reflectivity of the optical surfaces, and the optical path length between the two cavity surfaces. The result of the interference is that cones of light are formed by the etalon and directed to the PDA surface where the light generates the optical fringe pattern.
As the discussion above suggests, many laser systems now include an onboard spectrometer, sometimes referred to as a “wavemeter” for measuring bandwidth and/or center wavelength. Requirements for tighter wavelength and bandwidth control have increased the need to initially provide and thereafter maintain a close-tolerance alignment between the etalon and detector in the laser's onboard spectrometer. The initial alignment may, for example, be altered during shipment or installation of the laser, adversely affecting the precision of the wavemeter. Typically, the onboard wavemeter module is sealed, and in some cases purged. Thus, alignment inspections techniques that require physical access to the internal wavemeter components can be expensive and time-consuming.
With the above considerations in mind, Applicants disclose methods and apparatus for aligning an etalon with a photodiode array. In addition, methods and apparatus for inspecting an alignment between an etalon and a photodiode array are described.